1. Field of the Invention
The present invention relates to a steering damper provided for a steering mechanism of a vehicle to adjust a damping force on steering, a saddle riding type vehicle including the same, and a method of manufacturing the same.
2. Description of the Related Art
Conventionally, this type of apparatus has a lower casing, a first seal member, an electromagnet, a second seal member and an upper casing (see Unexamined Patent Publication No. 2010-254117 (FIGS. 4-9), for example).
This steering damper includes, as its basic construction, an upper casing having a ring-like receiving recess opening downward, and a lower casing inserted in the receiving recess from below to act as its relatively rotatable lid. An electromagnet and a magnetic fluid are contained in the receiving recess, and the receiving recess is sealed by two seal members. More particularly, the electromagnet is formed of a casing main body (32b) having a sideways U-shaped groove formed circumferentially thereof to be directed outward, and a coil wound around in this groove of the casing main body (32b). The electromagnet is attached to the lower casing (32a). The electromagnet and the lower casing are assembled through a bearing metal (35) to be rotatable relative to the upper casing. In this case, a seal member is disposed between relative rotation surfaces of the two casings to seal the receiving recess. A fluid receiving chamber is formed in a gap between an inner peripheral surface of the upper casing and an outer peripheral surface of the electromagnet, and a ceiling surface of the upper casing and an upper surface of the casing main body. The magnetic fluid is injected into this fluid receiving chamber.
With the steering damper constructed in this way, the lower casing is fixed to a head tube, and the upper casing is fixed to an upper bracket holding a steering handle. When the steering handle is turned and a magnetic field is generated by the electromagnet, the viscosity of the magnetic fluid will increase. Therefore, it can provide an effect of being able to inhibit vibration and wobbling in directions of rotation of the steering handle during driving, for example.
For the above steering damper, a manufacturing method is employed in which, after being assembled as described above and before being attached to the vehicle, the magnetic fluid is supplied from an inlet and the fluid receiving chamber is filled with the magnetic fluid while vacuuming from a discharge port communicating with the fluid receiving chamber. The vacuuming is carried out, since the viscosity of the magnetic fluid is very high, in order to make it easy to fill the magnetic fluid smoothly into the fluid receiving chamber formed in a narrow space. This is done also to leave as little air as possible in the magnetic fluid chamber.
However, the conventional example with such construction has the following problems.
That is, with the conventional apparatus, when the volume of the magnetic fluid expands with temperature increase, the internal pressure of the fluid receiving chamber will increase, which causes a possibility of the upper casing and the lower casing separating slightly from each other. When such a situation arises, air entrainment will be caused by air entering the fluid receiving chamber from the seals, thereby giving rise to the problem of adversely affecting the characteristics of the steering damper.
The conventional manufacturing method has a problem of complicated manufacturing because of the need for vacuuming.